Once, the expectations on reproduced audio sound were no greater than what monophonic reproduction could produce, and, accordingly, this satisfied most listeners. In course of time, however, the demand on high quality reproduction of stereo sound, e.g., recordings from a recording studio or recordings from a live concert, has been constantly increasing.
Consequently, various systems have been developed, each being able to reproduce true stereo sound to a greater or lesser extent.
The system that most readily comes to mind is a conventional system for stereophonic reproduction wherein left and right side speakers are disposed in front of a listener and with a certain distance separating the speakers. Most reproduction systems of today are based on this technology. However, true reproduction of the electrical stereo signal, both in terms of relative intensity between the sound waves perceived by the ears of the listener and the time difference between these, can at best be perceived only at one single position in relation to the loudspeakers, as these methods are often subject to incorrect translation of the electrical stereo information due to preferences of the separate loudspeakers and how the loudspeakers are positioned in relation to the listener.
The system coming closest to virtually move the listener to the recording location, i.e. to convey an impression of the true location of the different sound sources of the original event, is the binaural method of recording and the binaural method of reproduction (headphones). There are, however, a number of loudspeaker systems that introduce so called crosstalk cancellation by means of DSP, see, for example, U.S. Pat. No. 3,236,949 and U.S. Pat. No. 5,862,227. The purpose of such systems is to eliminate the signal that is reaching the left ear from the right speaker and vice versa. This is in order to create a binaural loudspeaker system. The disadvantage of such a system is that the complexity of the cross talk cancellation signal itself is degrading the sound quality. All other ways than the binaural method to record and reproduce sound, such as the above conventional two speaker set-up, is a creation of an imaginary sound image that is truly subjective and need not have even a remote resemblance with the actual experience at the recording position.
Consequently, there exists a need for a sound reproduction system that provides identical reproduction of the stereo sound image regardless of setup and quality of the loudspeakers. One such system that solves this problem is described in the patent application WO 01/39548, assigned to the applicant of the present invention, which discloses a method of processing and reproducing an input audio stereo signal. The system described in WO 01/39548 allows an audio stereo signal to be reproduced with a high degree of fidelity with high consistency in the perceived stereo image regardless of the quality of system.
A problem with such a system with closely located loudspeaker units, however, is that as the distance between loudspeaker units and listener increases, the performance of the system as regarding the fidelity in perceived stereo effect at the listeners location degrades with increasing distance and in the extreme case vanishes totally. Consequently, there exists a need for an improved system for reproducing sound.